Technical Field
[0001] The present invention relates to an impact tool such as an impact driver or an impact
wrench.
Background Art
[0002] An impact tool disclosed in
Japanese Patent Application Publication No. 2002-0 46078 drives a rotational impact system, with a battery pack as a power source and with
a motor as a driving source, so as to give a rotary motion to and an impact on an
anvil. The impact tool then intermittently transmits the rotational impact force to
an end bit to tighten a screw, and the like. A direct-current motor having a brush
and a commutator is known as a motor which has been employed as the driving source.
An impact tool including a motor having brushes is also disclosed in
US 4,412,158. This tool also includes a current limiter which reduces the motor speed if a preset
current level is exceeded. On the other hand, several attempts to employ a brushless
direct-current motor instead of the direct-current motor, is also made. Since brushless
direct-current motor is more excellent in torque characteristics than the direct-current
motor with brush, the impact tool that employs the brushless direct-current motor
can tighten a screw, a bolt, or the like, into a workpiece more powerfully.
Disclosure of Invention
Technical Problem
[0003] However, in order to tighten a member of hard material such as a bolt or a nut, a
large impact reaction force unavoidably occurs between an anvil and a hammer for hitting
the anvil. In addition to the impact reaction force, the driving force of the brushless
direct-current motor also moves the hammer backward to a large extent. If the hammer
moves backward to an excessive degree, a larger impact force is applied onto the system
facing the hammer due to the collision therebetween, thereby breaking the system.
Technical Solution
[0004] In view of the foregoing, it is an object of the present invention to provide an
impact tool which facilitates a tightening operation with a large torque, as well
as which prevents a system facing a hammer from breaking when a rotational impact
force occurs.
[0005] In order to attain the above and other objects, the present invention provides an
impact tool in accordance with claim 1, including a spindle, a motor, a rotational
impact system, a current detecting unit, and a current control unit. The spindle extends
in an axial direction thereof. The motor provides the spindle with a rotational power
in accordance with a motor current flowing therethrough. The rotational power rotates
the spindle about the axis at an rpm value. A rotational impact system provides an
impact force as set forth in claim 1, thereby transmitting both the rotational power
and the impact force to an end bit. The current detecting unit detects a current value
of the motor current. The current control unit reduces the current value if the current
value detected by the current detecting unit exceeds a predetermined value.
[0006] In this configuration, the impact by the spindle can be prevented from being excessive.
[0007] The current control unit reduces the current value during a first time period including
a timing at which the rotational impact system provides the spindle with the impact
force if the current value detected by the current detecting unit exceeds the predetermined
value.
[0008] In this configuration, the impact by the spindle can be effectively prevented from
being excessive.
[0009] Preferably, the impact tool further includes an rpm detecting unit configured to
detect the rpm value; and a minimum rpm determining unit configured to determine a
minimum rpm from a plurality of rpm values detected, during a second time period,
by the rpm detecting unit. The current control unit starts to reduce the current value
after a third time period has elapsed since the minimum rpm determining unit had determined
the minimum rpm value.
[0010] In this configuration, the time at which the impact occurs can be detected reliably.
[0011] Preferably, the impact tool further includes a maximum rpm determining unit configured
to determine a maximum rpm from the plurality of rpm values detected, during the second
time period, by the rpm detecting unit; and a period changing unit configured to change
the first time period based on a period after the maximum rpm is detected before the
minimum rpm is detected.
[0012] In this configuration, the intervals can be corrected even when the impact by the
spindle occurs at uneven intervals.
[0013] Preferably, the impact tool further includes an impact interval detecting unit configured
to detect an impact interval at which the rotational impact system hits the end bit
based on the period after the maximum rpm is detected before the minimum rpm is detected.
The period changing unit changes the first time period so that the first time period
becomes longer than a reference time period, if the impact interval detected by the
impact interval detecting unit is longer than a reference interval. The period changing
unit changes the first time period so that the first time period becomes shorter than
the reference time period, if the impact interval detected by the impact interval
detecting unit is shorter than the reference interval.
[0014] In this configuration, the intervals can be corrected reliably even when the impact
by the spindle occurs at uneven intervals.
[0015] Preferably, the current control unit reduces the current value if the current detecting
unit detects the current value exceeding the predetermined value a predetermined number
of times during a fourth time period.
[0016] In this configuration, the excessive impact by the spindle can be prevented reliably
from occurring.
[0017] Preferably, the current control unit maintains the current value if the current detecting
unit fails to detect the current value exceeding the predetermined value during a
fifth time period.
[0018] In this configuration, the current value is not reduced when it is not desirable
to reduce the current value. Therefore, a screw or the like can be securely tightened
in a wooden board or the like
[0019] Preferably, the motor is a brushless direct-current motor.
[0020] In this configuration, the impact tool can tighten a screw, a bolt, or the like,
into a workpiece more powerfully.
Advantageous Effects
[0021] With the invention described above, the impact by the spindle is prevented from being
excessive, thereby preventing the spindle from moving backward to an excessive degree
to crash into the opposite wall.
Brief Description of Drawings
[0022]
Fig. 1 shows a whole configuration of an electric tool according to embodiments of
the present invention;
Fig. 2 schematically illustrates the relation between an operation of a rotational
impact system included in the electric tool shown in Fig. 1 and a motor rpm;
Fig. 3 is a functional block diagram showing a motor driving control system of the
electric tool shown in Fig. 1;
Fig. 4 is a time chart showing various characteristics when a drive control according
to a first embodiment of the present invention is performed;
Fig. 5A is a flowchart illustrating the drive control according to the first embodiment
of the present invention;
Fig. 5B is a flowchart to be continued to the flowchart shown as Fig. 5A;
Fig. 6 is a time chart showing various characteristics when a drive control according
to a second embodiment of the present invention is performed;
Fig. 7A is a flowchart illustrating the drive control according to the second embodiment
of the present invention;
Fig. 7B is a flowchart to be continued to the flowchart shown as Fig. 7A;
Fig. 8 is a time chart showing various characteristics when a drive control according
to a third embodiment of the present invention;
Fig. 9A is a flowchart illustrating the drive control according to the third embodiment
of the present invention;
Fig. 9B is a flowchart to be continued to the flowchart shown as Fig. 9A;
Fig. 9C is a flowchart to be continued to the flowchart shown as Fig. 9B;
Fig. 10 is a time chart showing the relation between a motor current Ih under high
load, a motor current II under low load, and a threshold current Ith; and
Fig. 11 is a flowchart illustrating the drive control according to a fourth embodiment
of the present invention.
Explanation of Reference
[0023]
100 impact driver
1 brushless direct-current motor
2 inverter
3 control circuit section
31 operation unit
32 current detection circuit
33 applied voltage setting circuit
36 rotational speed detection circuit
37 control signal output circuit
10 rotational impact system
11 spindle
Best Mode for Carrying Out the Invention
[0024] Hereinafter, preferred modes of the present invention will be described with reference
to the accompanying drawings.
Mode for the Invention 1
[0025] Fig. 1 shows a whole configuration of an electric tool, in which the present invention
is applied to a cordless impact driver. Figs. 2 illustrate an operation of a rotational
impact system. Fig. 3 is a block diagram showing a configuration of a motor driving
unit of the electric tool which includes a brushless direct-current motor.
[0026] Referring first to Fig. 1, a configuration of an impact driver 100 according to modes
of the present invention is described. The impact driver 100 includes a tool body
which has a main body housing 6 extending from one end thereof (right in the figure)
to the other end (left in the figure), in the same direction (horizontal direction)
as the rotating shaft of a brushless direct-current motor 1 to be described later
(hereinafter, referred to as a "motor 1 "); and a handle housing 7 projecting downward
from the main body housing 6. An end bit holder 8 is provided at the other end of
the main body housing 6. Although not shown, a driver bit (end bit) is detachably
mounted to the end bit holder 8 so that a screw is tightened into a workpiece in the
use of the rotational impact force applied from the tool body. Instead of the driver
bit, a bolt-tightening bit can be mounted as an end bit.
[0027] To the one end of the main body housing 6, a motor 1 is mounted as a driving source.
At the other end of the main body housing 6, the end bit (not shown) is detachably
mounted to the end bit holder 8 for delivering rotational impact force.
[0028] On the side of the one end of the main body housing 6, a circuit board having an
inverter 2 for driving the motor 1, is mounted. At intermediate positions within the
main body housing 6, are mounted a power transmission system (speed reduction system)
9 for transmitting rotational power in the rotating shaft direction of the motor 1;
a rotational impact system 10 for producing the rotational impact force; and an anvil
13 for transmitting the rotational impact force of the rotational impact system 10
to the end bit.
[0029] To the bottom end of the handle housing 7, a battery pack case 4 which holds a battery
pack 4a is detachably mounted as a power source of the motor 1. Above the battery
pack case 4, a circuit board having a control circuit section 3 for controlling the
inverter 2 of the motor 1, extends in a direction across the figure. On the other
hand, a trigger switch 15 is provided at the top end of the handle housing 7. The
trigger switch 15 protrudes forward from the handle housing 7, in an urged state by
a spring. As will be described later, the trigger switch 15 is depressed into the
handle housing 7 against spring tension, thereby starting the motor 1. The rpm of
the motor 1 is controlled by adjusting the amount of pressing the trigger switch 15.
[0030] The battery pack 4a is electrically connected so that power is supplied to the trigger
switch 15 and the control circuit (circuit board) section 3, as well as to the inverter
section 2 at the same time.
[0031] The rotational power from the rotary output shaft of the motor 1 is transmitted to
a spindle 11 included in the rotational impact system 10, through the power transmission
system 9 engaging with the gear teeth of the rotary output shaft. The power transmission
system 9 includes a pinion gear (sun gear) 9a, and two planet gears 9b engaging with
the pinion gear9a. These gears are located in an inner cover (not shown) within the
main body housing 6. The power transmission system 9 transmits the rotational power
whose speed is reduced relative to that of the brushless direct-current motor 1, to
the spindle 11.
[0032] The rotational impact system 10 includes the spindle 11 to which rotational power
is transmitted through the power transmission system 9; a hammer 12 attached to the
spindle 11, engaging with the spindle 11 movably in the rotating shaft direction,
for producing rotational impact force; and an anvil 13 rotated by the rotational impact
force produced by the hammer 12, having the end bit holder 8. The hammer 12 has two
hammer projections (percussors) 12a. The anvil 13 has two anvil projections 13a. The
hammer projections 12a and the anvil projections 13a are symmetrically arranged at
two positions on a plane of rotation, in a manner such that each hammer projection
12a and its corresponding anvil projection 13a engages with each other in the rotating
direction.
[0033] The engagement between each projection pair of 12a and 13a transmits rotational impact
force. The hammer 12 is a ring-like frame surrounding the spindle 11 so as to be slidably
in contact with the spindle 11 in the shaft direction, and is in an urged state by
the spring 14 forward in the shaft direction. On the inner face of the hammer 12,
an inverted V-shaped (generally triangle) cam groove 12b is formed. On the other hand,
on the periphery of the spindle 11, a V-shaped cam groove 11a is formed in the shaft
direction. A ball (steel ball) 17 is inserted between the cam groove 11a and the cam
groove 12b formed on the inner face of the hammer 12 so that the hammer 12 through
the ball.
[0034] Fig. 2 shows the relation between a schematic operation of the rotational impact
system 10 and a motor rpm, in which (A) shows a state that the hammer 12 moves backward
and has left the projections 13a of the anvil 13; (B) shows a state that the hammer
12 rotatingly moves toward the projections 13a of the anvil 13, urged by a not shown
spring, from the backward position; and (C) shows a state immediately before the hammer
12 goes into engagement between the projections 12a of the hammer 12 and the projections
13a of the anvil 13 in order to give a rotational impact force to projections 13a
of the anvil 13 by the tension of the spring.
[0035] In the rotational impact system 10, if the torque produced between a workpiece and
a clamping part such as a screw, is not high excessively, the rotational power of
the spindle 11 given by the motor 1 is transmitted to the hammer 12 through the ball
17 held between the cam groove 11a of the spindle 11 and the cam groove 12b of the
hammer 12. As a result, the spindle 11 and the hammer 12 start rotating together.
The spindle 11 and the hammer 12 are twisted relative to each other. The hammer 12
twistingly compresses the spring 14 along the cam groove 11a of the spindle while
moving backward (direction of the arrow shown in (A) of Fig. 2). After the hammer
projections 12a leave the combination with the corresponding anvil projections 13a,
when the hammer 12 gets over the height of the anvil projections 13a, the hammer 12
go out of the engagement with the anvil 13 (state shown in (A) of Fig. 2). In this
case, the motor rotates at minimum speed among states in which the hammer 12 is out
of the engagement with the anvil 13. Furthermore, the hammer 12 rotatingly moves forward,
urged by the spring 14 and guided by the cam groove 11a (state shown in (B) of Fig.
2). The hammer projections 12a give impact torque to the anvil projections 13a of
the anvil 13 positioned in front of each hammer projection 12a in the rotating direction
(state shown in (C) of Fig. 2). The impact torque is transmitted to the driver bit
attached to the end bit holder 8 of the anvil 13. The driver bit then transmits the
impact torque to the clamping screw, thereby tightening the screw into the workpiece
or clamping the workpiece. This means that the hammer projections 12a and the anvil
projections 13a move into engagement again. After that, the hammer 12 starts moving
backward again, thereby repeating the above-described impact operation.
[0036] Referring next to Fig. 3, the inverter circuit section 2 of the motor 1 and the control
circuit section 3 are described.
[0037] In this mode, the motor 1 is a three-phase brushless direct-current motor. The motor
1 includes an inner rotor 1b having a permanent magnet including one pair of north
and south poles, embedded therein; three rotational position detectors (hall ICs)
5a, 5b, and 5c arranged at intervals of 60°, for detecting the rotational position
of the magnet rotor 1b; and an armature winding 1d having three-phase windings U,
V, and W of a star-connected stator 1c, controlled to become a current application
section of an electric angle of 120° based on position detection signals from the
rotational position detectors 5a, 5b, and 5c. In this mode, the motor 1 detects the
position of the rotor 1b by using the hall ICs in an electromagnetic coupling manner.
However, the rotor position can also be detected sensorlessly by extracting the induced
electromotive voltage (counter electromotive force) of the stator winding 1d as logical
signals, through a filter.
[0038] The inverter circuit section (power converter) 2 includes six, three-phase bridge-connected
FETs (hereinafter, referred to as "transistors") Q1-Q6; and a flywheel diode (not
shown). Each gate of the bridge-connected transistors Q1-Q6 is connected to a control
signal output circuit 37. Either source or drain of each of the six transistors Q1-Q6
is connected to one of the star-connected armature windings U, V, and W. A switching
element driving signal is inputted from the control signal output circuit 37 so that
the six transistors Q1-Q6 perform a switching operation. As a result, power is supplied
to the armature windings U, V, and W, with the direct-current voltage of the battery
pack 4a applied to the inverter 2 as three-phase (U-phase, V-phase, and W-phase) voltages
Vu, Vv, and Vw.
[0039] The control circuit section 3 includes an operation unit 31, a current detection
circuit 32, an applied voltage setting circuit 33, a rotating direction setting circuit
34, a rotational position detection circuit 35, a rotational speed detection circuit
36, and a control signal output circuit 37. The operation unit 31, although not shown,
has a microcomputer which includes a CPU for outputting driving signals based on processing
programs and data; a ROM for storing programs and control data corresponding to flowcharts
to be described later; a RAM for storing data temporarily; and a timer. The current
detection circuit 32 detects the motor current flowing through the motor 1. The detected
current is inputted to the operation unit 31.
[0040] The applied voltage setting circuit 33 sets the voltage to be applied to the motor
1, specifically, the duty ratio of a PWM signal, in response to the amount of the
pressure applied by the trigger switch 15. The rotating direction setting circuit
11 sets the rotating direction of the motor 1 by detecting an operation of rotating
the motor in either forward or reverse direction performed through a forward-reverse
switching lever 16. The rotational position detection circuit 35 detects the positions
of the rotor 1b and the stator 1c, relative to the armature windings U, V, and W,
based on signals outputted from the three rotational position detectors 5a, 5b, and
5c. The rotational speed detection circuit 36 detects the rpm of the motor, based
on the number of detection signals from the rotational position detection circuit
35, counted per unit time.
[0041] The control signal output circuit 37 transmits PWM signals to the transistors Q1-Q6
positioned on the power source side, based on the output from the operation unit 31.
The pulse width of each PWM signal is controlled so that power to be supplied to each
of the armature windings U, V, and W is adjusted, thereby controlling the rpm of the
motor 1 in the preset rotating direction.
[0042] Referring next to Figs. 4, 5A and 5B, a description is given for the control of an
impact driver 100 according to a first mode. Fig. 4 is a time chart showing the relation
between an impact torque T, a motor current I, and a motor rpm N. Fig. 5A and Fig.
5B are flowcharts showing the control of reducing the rpm of the motor 1 before and
after the impact by the hammer 12.
[0043] Referring first to Figs. 2 and 4, the relation between an impact torque, a motor
current, and a motor rpm, is described.
[0044] As the hammer 12 goes into engagement with the anvil projections 13a of the anvil
13, the load applied to the motor 1 reaches a maximum. As shown in Fig. 4, the rpm
N of the motor 1 reaches a minimum ((A)) in the result. On the other hand, since the
load applied to the motor 1 reaches a maximum, the motor current I reaches a maximum
((B)). After that, as the hammer 12 gets on the anvil projections 13a of the anvil
13, the load applied in the rotating direction of the motor 1 is reduced. The hammer
12 then gets over the anvil projections 13a of the anvil 13, to go out of the engagement
with the anvil 13 ((A) and (B) of Fig. 2). In this case, the load applied to the motor
1 reaches a minimum, and the rpm N of the motor 1 reaches a maximum ((C)). On the
other hand, since the load applied to the motor 1 reaches a minimum, the motor current
I reaches a minimum ((D)). The moment the rpm N of the motor 1 reaches a maximum with
the motor current I reaching a minimum, the hammer 12 performs an impact motion ((E)).
[0045] If a motor having a large drive power, such as a brushless motor, is employed in
this case, the impact by the hammer is too strong. When the hammer gets on the anvil
projections, the hammer moves backward to an excessive degree. This may cause the
hammer to crash into the opposite wall, thereby breaking the wall. In order to prevent
such a situation, the rpm of the motor 1 is reduced before and after the impact by
the hammer 12 in this mode.
[0046] Referring to the flowcharts of Figs. 5A and B, in S501, the CPU determines whether
or not the PWM duty of the motor control is 100%. This is because the hammer 12 usually
moves backward to an excessive degree when the trigger switch 15 is depressed to the
fullest extent, specifically, when the PWM duty cycle is 100%.
[0047] If the PWM duty cycle is not 100% (S501: NO), the CPU continues to determine whether
or not the PWM duty cycle is 100%. If the PWM duty is 100% (S501: YES), the CPU determines
whether or not the motor current I is 35A or larger in S502. In this mode, a threshold
value is set to 35A, which may cause the hammer 12 to move backward to an excessive
degree. However, another value can be employed as the threshold value.
[0048] If the motor current I is smaller than 35A (S502: NO), the CPU continues to determine
whether or not the motor current I is 35A or larger. If the motor current I is 35A
or larger (S502: YES), the CPU starts the timer for a time period Ta (10 msec) in
S503 (see Fig. 4). In S504, the CPU determines again whether or not the motor current
I is 35A or larger.
[0049] If the motor current I is 35A or larger (S504: YES), the CPU counts up a CNT 1 in
S505. In S506, the CPU determines whether or not the time period Ta (10 msec) has
passed. If the motor current I is smaller than 35A (S504: NO), the CPU determine whether
or not the time period Ta (10 msec) has passed, without counting up the CNT 1 in S506.
In this manner, the number of times the motor current I is equal to the threshold
value 35A or larger, is counted, detected within a predetermined period of time (10
msec in this mode).
[0050] If the time period Ta (10 msec) has not passed yet (S506: NO), the CPU returns to
S504 after a time interval of 1 msec in S507. In S504, the CPU again determines whether
or not the motor current I is 35A or larger. If the time period Ta (10 msec) has passed
(S506: YES), the CPU determine whether or not the number counted up by the CNT 1 is
larger than 5 in S508.
[0051] If the number counted up by the CNT 1 is 5 or smaller (S508: NO), the CPU returns
to S502. In S502, the CPU again determines whether or not the motor current I is 35A
or larger. If the number counted up by the CNT 1 is larger than 5 (S508: YES), the
CPU counts up a CNT 2 in S509. In S510, the CPU determines whether or not the number
counted up by the CNT 2 is larger than 5. If the number counted up by the CNT 2 is
5 or smaller (S510: NO), the CPU returns to S502. In S502, the CPU again determines
whether or not the motor current I is 35A or larger. After the determination five
times in S508, that the motor current I detected in S503 to S507 becomes equal to
or exceeds the threshold value 35A more than five times in total, the CPU starts the
control of reducing the rpm of the motor 1.
[0052] If the number counted up by the CNT 2 is larger than 5 (S510: YES), the CPU decides
the maximum value Nmax for the motor rpm N in S511 (see Figs. 4). In this mode, the
CPU detects the motor rpm N per 1 msec. If a detected result is larger than the previous
detected result, the CPU updates the maximum value. The CPU employs the updated value
after four detection operations as the maximum value Nmax. As a result, the CPU detects
the moment when the impact by the hammer 12 occurs.
[0053] In S512, the CPU decides a minimum value Nmin for the motor rpm N (see Figs. 4).
In this mode, the CPU detects the motor rpm N per 1 msec. If a detected result is
smaller than the previous detected result, the CPU updates the minimum value. The
CPU employs the updated minimum value after four detection operations as a minimum
value Nmin. As a result, the CPU detects the moment when the hammer 12 combines with
the anvil projections 13a, specifically, the moment immediately before the hammer
12 gets on the anvil projections 13a.
[0054] In S513, the CPU starts the timer for a time period Tb (7 msec). In S514, the CPU
determines whether or not the time period Tb (7 msec) has passed (see Figs. 4). If
the time period Tb (7 msec) has not passed yet (S514: NO), the CPU continues to determine
whether or not the time period Tb (7 msec) has passed. In this case, the time period
Tb (7 msec) is not limited to 7 msec as long as the time period Tb is shorter than
the time period after the moment when the hammer 12 engages with the anvil projections
13a, until the moment the impact by the hammer 12 occurs. As a result, the motor 1
is driven with a PWM duty cycle of 100% until the moment a little before the impact
by the hammer 12 occurs.
[0055] If the Tb (7 msec) has passed (S514: YES), the CPU starts the timer for a time period
Tc (6 msec) in S515. In S516, the CPU reduces the PWM duty cycle to 70% (see Fig.
4). In this case, the time period Tc (6 msec) is not limited to 6 msec as long as
the time period Tc includes the moment when the impact by the hammer 12. As a result,
the motor 1 is driven with a PWM duty cycle of 70% before and after the moment when
the impact by the hammer 12 occurs.
[0056] After that, the CPU determine whether or not the time period Tc (6 msec) has passed
in S517 (see Fig. 4). If the time period Tc (6 msec) has not passed yet (S517: NO),
the CPU continues to determine whether or not the time period Tc (6 msec) has passed.
If the time period Tc (6 msec) has passed (S517: YES), the CPU returns the PWM duty
cycle to 100% in S518.
[0057] This configuration reduces the PWM duty cycle of the motor control, specifically,
reduces the rpm of the motor 1, before and after the moment when the impact by the
hammer 12 occurs. As a result, the configuration prevents the impact by the hammer
12 from being excessive, thereby preventing the hammer 12 from moving backward to
an excessive degree to crash into the opposite wall. Further, since the PWM duty cycle
is reduced when the number at which the current value exceeds a predetermined value
is equal to or greater than a predetermined number, the excessive impact by the spindle
can be prevented reliably from occurring. Further, since the PWM duty cycle is reduced
after the minimum value of the motor rpm is detected, the time at which the impact
occurs can be detected reliably.
Mode for the Invention 2
[0058] Referring next to Figs. 6, 7A and 7B, a description is given for the control of an
impact driver 100 according to a second mode of the present invention. Figs. 6 are
time charts showing the relation between an impact torque T, a motor current I, and
a motor rpm N. Figs. 7A and 7B are flowcharts showing the control of reducing the
rpm of the motor 1 before and after the impact by the hammer 12. In Figs. 7A and 7B,
the steps which are the same as in the flowcharts of Figs. 5A and 5B have the same
reference numbers. A description is given only for different steps here.
[0059] In the second mode, after determining that the PWM duty cycle is 100% in S501 of
Fig. 7A, the CPU starts the timer for a time period Tz (300 msec) in S701 (see Figs.
6). After that, the CPU determines whether or not the time period Tz (300 msec) has
passed in S702. If the time period Tz (300 msec) has not passed yet (S702: NO), the
CPU proceeds to S502 to perform the control described in Figs. 5A and 5B. If the CPU
determines that the number counted up by the CNT 2 is 5 or smaller in S510, the CPU
returns to S702 to determine whether or not the time period Tz (300 msec) has passed.
On the other hand, if the CPU determines that the time period Tz (300 msec) has passed
(S702: YES), the CPU continues to determine whether or not the time period Tz (300
msec) has passed. The control described in Figs. 5A and Fig. 5B is not performed later
in this mode.
[0060] Thus, in the second mode, if the CPU does not start the control of reducing the rpm
of the motor 1 within a predetermined period of time (300 msec in this mode), the
CPU does not perform the control of reducing the rpm of the motor 1 later in the process,
either. For example, if a driver is employed as the end bit, a screw is to be tightened
into a wooden board or the like. Therefore, if the rpm of the motor 1 is reduced during
the screwing operation, the screw is likely not to reach the right position therefor.
However, in the second mode, if the CPU does not start the control of reducing the
rpm of the motor 1 within the predetermined period of time, the CPU does not perform
the control of reducing the rpm of the motor 1 later in the process, either. As a
result, a screw is securely tightened in a wooden board or the like.
Mode for the Invention 3
[0061] Referring next to Figs. 8 and 9A to 9C, a description is given for the control of
an impact driver 100 according to a third mode of the present invention. Fig. 8 are
time charts showing the relation between an impact torque T, a motor current I, and
a motor rpm N. Fig. 9A to Fig. 9C are flowcharts showing the control of reducing the
rpm of the motor 1 before and after the impact by the hammer 12. In Fig. 9A to Fig.
9C, the steps which are the same as in the flowcharts of Figs. 7A and 7B have the
same reference numbers. A description is given only for different steps here.
[0062] In the third mode, after determining that the number counted up by the CNT 2 is larger
than 5 in S510 of Fig. 9A, the CPU determines whether or not a Tc flag meaning that
the time intervals of the impact by the hammer 12 are longer and shorter alternatively,
as shown in Fig. 8A is zero in S901. If the Tc flag is zero (S901: YES), the CPU determines
whether or not Td_old4 < Td_old3, Td_old3 > Td_old2, Td_old2 < Td_old1, and Td_old1<
Td at the same time in S902. In this case, the Td_old4, the Td_old3, the Td_old2,
and the Td_old1 mean Tds one to four cycles before, respectively. The term Td is described
later.
[0063] If Td_old4 < Td_old3, Td_old3 > Td_old2, Td_old2 < Td_old1, and Td_old1<Td at the
same time (S902: YES), the CPU sets the Tc flag to one in S904. After that, the CPU
decides the maximum value Nmax for the motor rpm N in S511. If NO in S901 or S902,
the CPU proceeds straight to S511 to decide a maximum value Nmax for the motor rpm
N.
[0064] Specifically, only when Td_old4 < Td_old3, Td_old3 > Td_old2, Td_old2 < Td_old1,
and Td_old1 < Td at the same time in a state that the Tc flag has been originally
set to zero, the CPU sets the Tc flag to one.
[0065] After deciding the maximum value Nmax for the motor rpm N in S511, the CPU starts
the timer in S904. The CPU then decides a minimum value Nmin for the motor rpm N in
S512. While deciding the minimum value Nmin for the motor rpm N, the CPU stops the
timer from counting, and stores the counted value Td in S905. Specifically, the counted
value Td means the period of time lapsed after the maximum value Nmax of the motor
rpm N until the minimum value Nmin thereof. The Td thus stored is used for making
the determination in S902. Therefore, the situation of S902 "Td_old4 < Td_old3, Td_old3
> Td_old2, Td_old2 < Td_old1, and Td_old1 < T at the same time" means that the time
intervals of the impact by the hammer 12 are longer and shorter alternatively, as
shown in Fig. 8A.
[0066] If the CPU determines that the time period Tb (7 msec) has passed in S513 and S514,
the CPU determines whether or not the Tc flag is one in S906. If the Tc flag is one
(S906: YES), the CPU determines whether or not the previous value of the Tc is 4 msec
in S907. If the previous value of the Tc is 4 msec (S907: YES), the CPU sets the time
period Tc to 9 msec in S908, and then starts the timer in S911. On the other hand,
if the previous value of the Tc is not 4 msec (S907: NO), the CPU sets the time period
Tc to 4 msec in S909, and then starts the timer in S911.
[0067] If the Tc flag is not one (S906: NO), the CPU sets the time period Tc to 6 msec in
S910, and then starts the timer in S911. In S912, the CPU reduces the PWM duty cycle
to 70% at the same time as the timer starts in S911. After that, in S913, the CPU
determines whether or not the time period Tc has passed.
[0068] If the time period Tc has not passed yet (S913: NO), the CPU continues to determine
whether or not the time period Tc has passed. If the time period Tc has passed (S913:
YES), the CPU returns the PWM duty cycle to 100% in S914. In S915, the CPU determines
whether or not a time period Tx has passed. If the time period Tx has not passed yet
(S915: NO), the CPU returns to S901 to determine again whether or not the Tc flag
is zero. If the time period Tx has passed (S915: YES), the CPU sets the Tc flag to
zero in S916, then return to S901.
[0069] In this mode, as described above, based on the past increase-decrease pattern of
the Td (impact intervals), the Td subsequent to the past Td is predicted. The subsequent
Td is controlled to have even impact intervals. Therefore, even when the impact by
the hammer 12 occurs at uneven intervals, the intervals can be corrected. This configuration
prevents the impact by the hammer 12 from being excessive, thereby preventing the
hammer 12 from moving backward to an excessive degree to crash into the opposite wall.
Mode for the Invention 4
[0070] Referring next to Figs. 10 and 11, a description is given for the control of an impact
driver 100 according to a fourth mode of the present invention. Fig. 10 is a time
chart showing the relation between a motor current Ih under high load, a motor current
I1 under low load, and a threshold current Ith. Fig. 11 is a flowchart showing the
control of reducing the motor current I when the motor current I exceeds the threshold
current Ith. In this mode, the motor current I is reduced when the motor current I
exceeds the threshold current Ith, like the motor current 1h under high load shown
in Fig. 10.
[0071] Referring to the flowchart of Fig. 11, in S1101, the CPU determines whether or not
the PWM duty cycle of the motor control is 100%. This is because the hammer 12 usually
moves backward to an excessive degree when the trigger switch 15 is depressed to the
fullest extent, specifically, when the PWM duty cycle is 100%.
[0072] If the PWM duty cycle is not 100% (S1101: NO), the CPU continues to determine whether
or not the PWM duty cycle is 100%. If the PWM duty cycle is 100% (S1101: YES), the
CPU determines whether or not the motor current I is 35A or larger in S1102. In this
mode, the threshold current Ith is set to 35A, which may cause the hammer 12 to move
backward to an excessive degree. However, another value can be employed as the threshold
current Ith.
[0073] If the motor current I is smaller than 35A (S1102: NO), the CPU continues to determine
whether or not the motor current I is 35A or larger. If the motor current I is 35A
or larger (S1102: YES), the CPU reduces the PWM duty cycle to 85% in S1103. As a result,
the motor 1 is driven with a PWM duty cycle of 85%.
[0074] After a time interval (3 msec) as a sampling time for controlling the operation unit
31 (S1104), the CPU increases the PWM duty cycle by 3% in S1105. In S1106, the CPU
determine whether or not the PWM duty cycle is 100% or larger. Although the PWM duty
cycle never exceeds 100% in practice, the CPU determine whether or not the PWM duty
cycle is 100% or larger on calculation in the operation unit 31.
[0075] If the PWM duty cycle is smaller than 100% (S1106: NO), the CPU returns to S1104.
After the time interval, the CPU increases the PWM duty cycle by 3% again in S1105.
If the PWM duty cycle is 100% or larger (S1106: NO), this means that the PWM duty
cycle has been set to 100%. The CPU returns to S1102 to determine again whether or
not the motor current I is 35A or larger.
[0076] In this configuration, if the motor current 1 exceeds the threshold current Ith,
the CPU reduces the motor current I. As a result, this configuration prevents the
impact by the hammer 12 from being excessive, thereby preventing the hammer 12 from
moving backward to an excessive degree, to crash into the opposite wall.
Industrial Applicability
[0077] An impact tool of the present invention can be used to tighten a screw, a bolt, or
the like, in a workplace.